Metal halide discharge lamps, which are characterized by high efficacy and superior color rendering index (CRI), are more and more widely used for general lighting. Until now, almost all metal halide discharge lamps used for general lighting have been operated at rated lamp power. The major reason commercial metal halide discharge lamps are operated at rated lamp power without dimming is that lamp correlated color temperature (CCT) and hue, Duv, change dramatically under dimming conditions. This limitation prevents metal halide discharge lamps from being used in many installations where occupancy sensor, daylight coupling and/or constant light output features are required.
Due to the ever increasing cost of energy and increased interest in energy-conserving lighting systems, some metal halide discharge lamp systems with dimming ballasts are available on the market. Under dimmed conditions, usually dimmed to 50% of rated lamp power, the color performance of the metal halide discharge lamps with the conventional dimming ballasts deteriorates dramatically. When the lamps are dimmed, the CCT of the lamps typically increases significantly, while the hue of the light deteriorates significantly away from white light. Furthermore, for many real applications, 50% of rated lamp power is still too high for both light output level and energy consumption. Dimming of metal halide discharge lamps to even lower power levels is desired.
Under dimming conditions, metal halide arc discharge chamber wall temperatures and coldest spot temperatures are lower than the temperatures at rated power due to the power reduction. At the lower coldest spot temperature under dimming conditions, the vapor pressure of the metal halide fill in the arc discharge chamber is reduced, causing significant changes of the CCT of the lamp.
U.S. Pat. No. 6,717,364, issued Apr. 6, 2004 to Zhu et al., discloses metal halide lamps with a special chemical fill which exhibit superior dimming characteristics. The disclosed lamps have improved performance compared to conventional metal halide lamps when the lamp power is 50% or more of the rated lamp power. Upon dimming below 50% of the rated lamp power, the CCT and hue of the lamps change significantly due to the further decrease of the coldest spot temperature of the arc discharge chamber.
U.S. Pat. No. 6,242,851, issued Jun. 5, 2001 to Zhu et al., discloses metal halide lamps that have significantly better lamp performance under dimming conditions to 50% of rated lamp power. A lamp has an arc discharge chamber in a vacuum outerjacket to reduce convection heat loss from the coldest spot of the arc discharge chamber, and a metal heat shield is used on the arc discharge chamber to reduce radiation heat loss from the coldest spot during lamp operation. The disclosed lamp exhibits very good dimming performance to 50% of the rated lamp power. However, widely used high voltage starting pulses on metal halide lamps in conjunction with a vacuum jacket may make the lamps susceptible to arcing when the arc discharge chamber leaks or slow outer jacket leaks exist. The vacuum outer jacket and metal shield at the coldest spot may keep the coldest spot temperature too high at rated wattage and can accelerate corrosion of the arc discharge chamber.
U.S. Pat. No. 5,698,948, issued Dec. 16, 1997 to Caruso, discloses a discharge lamp that contains halides of Mg, Ti and one or several of the elements from the group including Sc, Y and Ln. The lamp fill also contains Mg to improve lumen maintenance. The lamp has a disadvantage of a strong green hue when dimmed to lower than the rated power, due to the relatively high vapor pressure of TlI under dimming conditions.
U.S. Pat. No. 6,369,518, issued Apr. 9, 2002 to Kelly et al., discloses high intensity discharge lamps with electronic control of color temperature and color rendering index by changing the duty cycle of the alternating current waveform on the electrodes. The waveform of each cycle is modified to energize one electrode as positive or negative for a longer time than the other electrode, thereby altering the temperature distribution within the arc tube, whereby the coldest spot and hottest spot temperature in the arc tube are changed to provide a color variable metal halide discharge lamp. The patent mentions in a general sense that color temperature can be controlled during dimming.
Disadvantages of existing metal halide discharge lamps and dimmable ballast systems are as follows. Existing systems are designed for controlling lamp power only, with no consideration of color performance under dimming conditions. When lamp power is reduced from rated wattage to 50% of the rated wattage, the CCT of the lamp increases dramatically, often more than 1,000 K due to the decrease of the coldest spot temperature of the arc discharge chamber. These changes are not acceptable for most lighting applications. When lamp power is reduced from rated wattage to 50% of rated wattage or lower, the light radiated by a metal halide discharge lamp has a color point which is far away from the black body line, leading to a nonwhite hue.
Accordingly, there is a need for high intensity discharge lamp systems with improved dimming performance.